1. Field of the Invention
This invention pertains to a technique for multiplexing several different wavelength signals in Optical Slip Ring Assemblies. More distinctively, this invention pertains to an efficient optical power combiner technique for multiplexing several signals of different wavelengths to a single optical fiber.
2. Description of the Prior Art
Electrical signals are multiplexed together to form a large-bandwidth composite signal which modulates a light source. This method of multiplexing necessitates sophisticated electronic circuitry. Space division multiplexing which does not attempt to utilize the bandwidth capability of the fiber. Time division multiplexing send several signals over a common fiber by sampling each signal at a precise rate. This multiplexing scheme imposes severe requirements for linearity and high output power on the optical source. Wavelength division methods of multiplexing, currently in use or under research, employ techniques for sending several signals over a common fiber via its own wavelength. The primary components utilized for these methods are prism, mirrors, filters, lenses and gratings. The cost, size and reproducibility of these components are directly proportional to the performance.
Many different types of wavelength division multiplexing techniques have been proposed to multiplex several different wavelengths signals. Generally, the number of signals which can be multiplexed is limited to two or three different signals. The use of a bifurcated fiber optic bundles for both the rotating bundle and the non-rotating bundle in an optical slip ring asssemly was suggested in U.S. Pat. Nos. 4,027,945; 4,109,997 and 4,109,998. A bifurcated fiber can multiplex only two different wavelength signals. Several bifurcated fibers can be used to form a bundle which will require the technique of this invention (or a similar technique) to multiplex the set of dual wavelength signals to a single transmission line.
Another method for transferring light signals from a rotating body to a non-rotating body in an optical slip ring assembly is also represented by U.S. Pat. No. 4,027,945. Light carrying members may be bundles of fiber optics which are placed end to end coaxially with the axis of rotation. Multichannel bundles may be used where the channels at the coupled ends are each formed into concentric circles separated by opaque material. Although such methods have served the purpose, they have not proven to be entirely sufficient for a number of applications because of the limitation on the number of signals to be multiplexed; a problem which is overcome by the present invention.
Symmetric couplers (multiplexers) have been used to combine light signals in different fibers to a single trunk fiber of the same size. The overall cumulative insertion loss would drastically increase when several sources are multiplexed using this type system. An optical power combiner, fabricated by wrapping two smaller fibers around a central large trunk fiber, can multiplex three sources to a single trunk with an insertion loss between any of the three input fibers and the trunk of approximately 1 dB. As the number of smaller fibers increase the fabrication process becomes more difficult and the inertion-loss in each small fiber increases.
The new multiplexing dual wavelength LEDs and demultiplexing photodetectors are directed toward simpler wavelength division multiplexing systems. The source emits two wavelengths bands (modulated independently) from a single source. The demultiplexing detector can detect and separate signals from the two wavelength bands simultaneously. Multiple wavelength devices for multiplexing more than two wavelength signals are yet in the developmental stages.
In many different gimbal and rotating systems it is desirable that many different signals be carried at different wavelengths so that a single optical transmission line may be employed to multiplex a plurality of signals. There is therefore a need for a multiplexing technique for optical slip rings in which the number of signals to be multiplexed is significantly increased beyond the number known in the prior art. A technique is provided in the present invention for increasing the number of signals to be multiplexed without the use of bulk optics. The alignment accuracy, which is critical because of the small dimensions of single fibers, is greatly enhanced in the present invention. The large core fiber containing the combined signals can be used in an optical slip ring assembly in the same way as a single fiber containing one wavelength is used.